Heat transfer into a fluid involves the transfer of energy caused by a temperature differential between two bodies. For example, a heat exchanger design for heating a fluid is typically constructed of a highly conductive material, such as metal tubing containing a fluid flowing through the inside, wherein the tubing is heated from the outside. The heat transfers through the metal tubing by conduction, into the liquid through the boundary layer by conduction, and finally into the bulk fluid by convection.
Efforts to improve heat transfer include plate and frame heat exchangers, shell and tub heat exchangers, and a variety of different fin configurations for radiating or absorbing heat more efficiently.
Additionally, improvements based on fluid flow have been implemented to accelerate heat transfer. Examples include co-current flow, where the two hottest and coldest points of fluids flow together in the same direction in a heat exchanger. In contrast, counter flow devices flow the hottest and coldest points of fluids in opposite directions, which produces the greatest temperature differential between the two fluids. The greater temperature differential results in higher heat transfer efficiency for heat exchanger.
Fluid velocity has a significant effect on heat transfer. For example, laminar flow heat transfer mechanics have a lower heat transfer than turbulent flow heat exchangers. Therefore, turbulent heat transfer exchangers are preferred over laminar flow heat exchangers if the material being heated can withstand turbulent flow without degradation and if turbulent flow is economically feasible.
All flowing fluids have a wall effect or a boundary layer effect where the fluid velocity is greatly reduced at the point of contact with the wall of a vessel, such as a tube. The reduced fluid velocity hinders heat transfer efficiencies.
A further explanation of the boundary layer follows. Aerodynamic forces depend in a complex way on the viscosity of a fluid. As a fluid moves past an object, the molecules adjacent to the surface of the object stick to that surface. The flowing molecules of fluid just above the surface of the object are slowed by in their collisions with the fluid molecules that are sticking to the surface. These slowed molecules, in turn, slow down the flow just above them. The greater the distance away from the surface of the object, the fewer are the number of collisions that are affected by the surface of the object. This phenomenon results in a thin layer of fluid near the surface wherein the velocity changes from zero at the surface to the free stream value at a distance away from the surface. This thin layer is referred to as the boundary layer because the layer occurs on the boundary of the fluid.
As an object moves through a fluid, or as a fluid moves past an object, the molecules of fluid near the object are disturbed and move around the object. Aerodynamic forces are generated between the fluid and the object. The magnitudes of these forces depend on the shape of the object, the speed of the object, the mass of the fluid passing by the object. Additionally, two other important properties of the fluid affect the magnitude of aerodynamic forces, i.e., the viscosity, or stickiness, and the compressibility, or springiness, of the fluid. To properly model these effects, aerospace engineers use similarity parameters, which are ratios of these effects to other forces present in the problem. If two experiments have the same values for the similarity parameters, then the relative importance of the forces are being correctly modeled.
FIG. 1 shows the streamwise velocity variation from free stream to the surface. In reality, the effects are three dimensional. From the conservation of mass in three dimensions, a change in velocity in the streamwise direction causes a change in velocity in the other directions as well. As explained above, there is a small component of velocity perpendicular to the surface that displaces or moves the flow above it. The thickness of the boundary layer can be defined to be the amount of this displacement. The displacement thickness depends on the Reynolds number, which is the ratio of inertial (resistant to change or motion) forces to viscous (heavy and gluey) forces, is given by the equation:Reynolds number (Re) equals velocity (V) times density (r) times a characteristic length (l) divided by the viscosity coefficient (mu), i.e., Re=V*r*l/mu. 
As can be seen in FIG. 1, boundary layers may be either laminar, i.e., layered, or turbulent, i.e., disordered, depending on the value of the Reynolds number. For lower Reynolds numbers, the boundary layer is laminar and the streamwise velocity changes uniformly as one moves away from the wall, as shown on the left side of FIG. 1. For higher Reynolds numbers, the boundary layer is turbulent and the streamwise velocity is characterized by unsteady, i.e., changing with time, swirling flows inside the boundary layer. The external flow reacts to the edge of the boundary layer just as it would to the physical surface of an object. Therefore, the boundary layer gives any object an “effective” shape, which is usually slightly different from the physical shape. The boundary layer may lift off or “separate” from the body and create an effective shape much different from the physical shape. Flow separation occurs because the flow in the boundary layer has very low energy relative to the free stream, and is more easily driven by changes in pressure. Flow separation is the reason for airplane wing stall at high angle of attack. The effects of the boundary layer on lift are expressed mathematically by lift coefficient and the effects on drag by a drag coefficient.
The portion of a fluid flow near a solid surface is the portion where shear stresses are significant and inviscid-flow assumption may not be used. All solid surfaces interact with a viscous fluid flow because of the no-slip condition, which is a physical requirement that the fluid and solid have equal velocities at their interface. Thus, a fluid flow is retarded by a fixed solid surface and a finite, slow-moving boundary layer is formed. For a boundary layer to be thin, the Reynolds number of the body must be large, i.e., 103 or more. Under these conditions, the flow outside the boundary layer is essentially inviscid and plays the role of a driving mechanism for the layer.
Referring now to FIG. 2, a typical low-speed or laminar boundary layer is shown in the illustration. Such a display of the streamwise flow vector variation near the wall is called a velocity profile. The no-slip condition requires that u(x,0)=0, as shown, where u is the velocity of flow in the boundary layer. The velocity rises monotonically with distance y from the wall, finally merging smoothly with the outer, i.e., inviscid, stream velocity U(x). At any point in the boundary layer, the fluid shear stress τ, is proportional to the local velocity gradient, assuming a Newtonian fluid. The value of the shear stress at the wall is most important, since the value of shear stress relates not only to the drag of the body but often also to its heat transfer. At the edge of the boundary layer, τ approaches zero asymptotically. There is no exact spot where τ=0, therefore the thickness δ of a boundary layer is usually defined arbitrarily as the point where u=0.99U.
Recently, technologies have been developed that utilize nano-sized particles. Some of these technologies have focused on increasing heat transfer of a fluid or gas by increasing conductivity through the use of nano-powders. Such nano-powders are typically made from metals or ceramics. However, the use of such powders typically increase the viscosity of the fluid, resulting in an increased boundary layer, which tends to reduce the potential heat transfer gains.
Nanoparticles are made in many different ways they can be milled, chemically grown, precipitated out of a fluid through the reaction process or by other processes. These are only a few methods of manufacturing nano materials in industry that is in its infancy and growing rapidly.
Nanoparticles have had two processing problems which are: 1. The necessity of mechanical mixing to break up nano particle conglomerations and disperse nanoparticles homogeneously throughout a fluid; and 2. Once the nanoparticles have been suspended throughout the fluid, maintaining these particles in a stable precipitation over a prolonged time is problematic due to a tendency of the nanoparticles to settle out and re-conglomerate.
Techniques used to address the difficulty of nano material dispersion and prolonged stabilization include highly specialized surfactants, surface coatings and a variety of different mechanical mixing process.
The highly specialized surfactants that are used for nano dispersion have became their own unique specialty over the last 10 years and someone skilled of the surfactants can help choose the appropriate one for a variety of applications.
Additionally, there has been a large development of nano coatings that can produce surface effects such as: hydrophobic, hydrophilic, polar, nonpolar, negative and positively charged surfaces including adding functional groups.
Typical thermally conductive fluids in which nanoparticles are suspended include water, aqueous brines, mixtures of water with at least one of the group consisting of alcohols, glycols, and ammonia, hydro carbons, mineral oils, natural oils, synthetic oils, fats, waxes, ethers, esters, glycols, halogen derivatives of at least one of the group consisting of hydrocarbons, mineral oils, natural oils, synthetic oils, fats, waxes, ethers, esters, and glycols, 40 silicate esters, biphenyl, polyaromatic compounds, salt-hydrates, organic eutectics, clathrate-hydrates, paraffins, inorganic and organic eutectic mixtures, and combinations as set forth in U.S. Pat. No. 7,390,428 to Davidson et al for “Compositions with nano-particle size conductive material powder and methods of using same for transferring heat between a heat source and a heat sink”.
Some nano materials research over the last 15 years has been directed to thermal conductivity in fluids. For example, U.S. Pat. No. 6,695,974 to Withers for “Nano carbon materials for enhancing thermal transfer in fluids” teaches that the addition of metal and oxide nanoparticles that are small enough to remain in suspension in a fluid can substantially enhance the thermal conductivities of the fluid and thus substantially enhance heat transfer. The smaller the particle size the greater the effect of increasing the nanofluid thermal conductivity as well as the higher the thermal conductivity of the nanoparticle. For example, the thermal conductivity of a nanoparticle copper in a fluid provides a higher thermal conductivity than aluminum oxide because copper metal has a higher thermal conductivity than aluminum oxide.”
Heat transfer from a surface through the boundary layer of a fluid can also be improved by imperfections in the surface of a body. As an example, almost everyone who has ever cooked pasta has watched water boil in a pot and has noticed a peculiar phenomenon, namely that bubbles tend to form in one area on the bottom of the pan consistently. The usual assumption is that the bubble forming area is a hot spot in the burner or is a thinner region in the pan. Those assumptions are plausible. However, if the pan is turned or moved on the cooking surface the bubble forming areas may still produce bubbles more consistently than other areas.
What may be overlooked when watching the bubbles form in a bubble forming region of a pan is that there is usually a small surface deformity that creates a low surface energy point that allows bubbles to continuously form in that spot or region.
Experimental studies of critical heat flux enhancements using nanofluids under convection flow conditions have been performed, as discussed in “Experimental study of critical heat flux enhancement during forced convective flow boiling of nanofluid on a short heated surface”, by Ho Seon Ahn, Hyungdae Kim, HangJin Jo, SoonHo Kang, WonPyo Chang, Moo Hwan Kim, International Journal of Multiphase Flow 36 (2010) 375-384, incorporated herein by reference.
Previous studies have suggested that a likely critical heat flux (CHF) enhancement mechanism for nanofluids is an improvement in the ability of the fluid to wet the surface due to a thin nanoparticle sorption layer formed by evaporation of a nanoparticle containing microlayer beneath a bubble growing at the heated surface. Recently studies have focused on convective flow boiling of nanofluids in a circular stainless steel tube using the electrical heating. The studies reported significant increases in flow boiling critical heat flux of nanofluids with alumina, diamond, and zinc oxide that the contact angle on the tube decreased to control the concentration of nanofluid. Also, they found, in higher concentrations of nanofluid, that the critical heat flux enhancement was higher whereas the static contact angle on the fouled surface was lower. It was concluded from the experiments that the improved surface wettability due to the nanoparticles deposition layer caused significant critical heat flux enhancements during the convective flow boiling of nanofluids. The findings were consistent with previous pool boiling research.
Early research showed that a small surface change characteristic increases heat transfer by use of the nano fluid through a phenomena of nano plating of stagnant nanoparticles film on a surface. Even though surface plating phenomena caused by the electrical heating coils was accidental, the experiment resulted in a greater heat transfer.
One hypothesis was that the plausible reason for the changes in boiling heat transfer performance was the nanoparticle deposition onto the surface. Deposition was confirmed by a surface roughness measurement after the nanofluid boiling tests and the consequent change in nucleate site density. Pool boiling critical heat flux experiments of pure water on a nanoparticle-fouled heater as a result of a pre-boiling in nanofluid, showed an interesting result that the same magnitude of the significant critical heat flux increase in the nanofluid was observed for the nanoparticle-fouled surface submerged even in pure water.
This solution for increasing critical heat flux seems simple, i.e., just produce nano plating surfaces on piping/and tubing for commercial use. There are two problems with this solution, however. The first is that it is not cost-efficient to produce nano composite surfaces. The second problem relates to the fact that, in real applications, heat transfer surfaces tend to become fouled which, would reduce the efficiency of the nano plating.
As set forth in “Experimental Study of Critical Heat Flux Enhancement During Forced Convective Flow Boiling of Nanofluid on a Short Heated Surface”, by Ho Seon Ahn, Hyungdae Kim, HangJin Jo, SoonHo Kang, WonPyo Chang, Moo Hwan Kim, International Journal of Multiphase Flow 36 (2010) 375-384, which is hereby incorporated by reference, it was found that adding tiny amounts (less than 0.001% by volume) of alumina nanoparticles to a conventional cooling liquid could significantly increase the critical heat flux (CHF) up to 200%. The large critical heat flux enhancement in nanofluids were attributed to the surface wettability effect, which was induced by nanoparticles deposition by boiling of the fluid.
Finally, difficulties associated with conducting heat into a flowing fluid have been attributed to the existence of a “film” of gas that is closely adherent to a metal surface when conducting heat into or out of a gas. As can be seen in U.S. Pat. No. 2,690,051 to Peskin, various attempts have been made to overcome the resistance to conduction of heat through the film. However, these efforts have mainly consisted of expedients for increasing the velocity and turbulence of the gas in the neighborhood of the heated surfaces. Some gains have been made in that way, but the film still remains the greatest impediment to heat transfer.